In these years, light and thin displays have been in high demand because light and thin personal computers and television receivers have appeared. In response to the demand, flat-panel displays such as liquid crystal displays (LCD) have been developed for replacing cathode ray tubes (CRT).
An LCD is a display in which an electric field is applied to an anisotropic dielectric liquid crystal layer between two substrates, and a desired video signal is obtained by controlling an amount of light passing through the substrates by adjusting the intensity of the electric field. Such an LCD is typically adopted as a simple flat-panel display for a mobile device. In particular, a TFT LCD adopting thin film transistors (TFT) as switching elements is commonly used.
Since nowadays LCDs are adopted not only to a display apparatus of a computer but also to a display apparatus of a television receiver, the LCDs are often required to support moving images. However, conventional LCDs cannot easily support moving images on account of a slow response speed.
To solve this problem of the response speed of liquid crystal, there has been a known method of driving liquid crystal (e.g. Japanese Laid-Open Patent Application No. 365094/1992), the method being arranged in such a manner that, in accordance with the combination of an input video signal of the directly previous frame and an input video signal of the current frame, a drive voltage that is higher (in the case of overshoot) or lower (in the case of undershoot) than a predetermined gray level voltage of an input video signal of the current frame is supplied to a liquid crystal display panel. Hereinafter, this driving method is termed “overshoot (OS) drive”.
It has been known that the response speed of liquid crystal is temperature-dependent to a great extent. In this relation, there is a liquid crystal panel driving apparatus that always keeps the response speed of gray level variation to be optimum and maintains the quality of displayed images, even when the temperature of the liquid crystal panel varies. Such a liquid crystal panel driving apparatus is disclosed by, for instance, Japanese Laid-Open Patent Application No. 318516/1992.
In reference to FIGS. 15-19, the following will describe the aforesaid apparatus that performs overshoot drive in order to correct the optical response characteristics of the liquid crystal panel, in accordance with the working temperature. Note that, FIG. 15 is a block diagram showing a substantial part of a conventional liquid crystal display apparatus, FIG. 16 illustrates an example of the contents of an OS table memory, FIG. 17 is a functional block diagram showing the outline of a control CPU, FIG. 18 illustrates the relationship between a temperature in the apparatus and a lookup table memory, and FIG. 19 illustrates the relationship between a voltage applied to liquid crystal and the response of the liquid crystal.
FIG. 15 illustrates the following members: members 1a through 1d are OS table memories (ROMs) storing respective OS parameters (enhancing conversion parameters) each corresponding to gray level transition around one frame period of input image data, the OS parameters being stored in such a manner as to one-to-one correspond to respective temperature ranges in the apparatus; a member 15 is a frame memory (FM) that stores input image data for one frame; and a member 14H is an enhancing conversion section that (i) compares input image data (current data) of an M-th frame to be displayed next with input image data (previous data) of an (M−1)-th frame, the previous data having been stored in the frame memory 15, (ii) reads out an OS parameter that corresponds to the result of the aforesaid comparison (gray level transition), from one of the OS table memories (ROMs) 1a through 1d, and (iii) determines, in accordance with the OS parameter having bean read out, enhancing conversion data (writing gray level data) required for the image reproduction in the M-th frame.
In addition to the above, FIG. 15 also illustrates the following members: a member 16 is a liquid crystal controller that outputs a liquid crystal drive signal to a gate driver 18 and a source driver 19 of a liquid crystal panel 17, in accordance with the enhancing conversion data supplied from the enhancing conversion section 14H; a member 20 is a temperature sensor for detecting a temperature in the apparatus; and a member 12H is a control CPU that outputs a switching control signal for switching the OS parameters used for the enhancing conversion of image data, the switching being performed by selecting and referring to one of the OS table memories (ROMs) 1a through 1d, with reference to the temperature detected by the temperature sensor 20.
The OS parameters LEVEL 1 through LEVEL 4 stored in the respective OS table memories (ROMs) 1a through 1d are obtained in advance from actual values of the optical response characteristics of the liquid crystal display panel 17, under reference temperatures T1, T2, T3, and T4 (T1<T2<T3<T4). The order of the degrees of enhancing conversion is LEVEL 1>LEVEL 2>LEVEL 3>LEVEL 4.
For instance, when the number of display signal levels, i.e. the number of sets of display data is 256 gray scales for 8 bits, the OS table memories (ROMs) 1a through 1d may have OS parameter values (actual values) for all 256 gray levels. Apart from this, the following arrangement shown in FIG. 16 may be adopted: the OS table memories (ROMs) 1a through 1d only store 9×9 OS parameter values (actual values) each representing 32 gray levels, and the sets of enhancing conversion data for the remaining gray levels are calculated by performing, for instance, linear interpolation using the aforesaid actual values. This arrangement makes it possible to reduce the storage capacity of the OS table memories (ROMs).
As shown in FIG. 17, the control CPU 12H includes: a threshold discriminating section 12a that compares the temperature detection data produced by the temperature sensor 20 with predetermined threshold temperature data values Th1, Th2, and Th3; and a control signal output section 12b that generates and outputs a switching control signal that switches the OS parameters LEVEL 1 through LEVEL 4 by selecting one of the OS table memories (ROMs) 1a through 1d with reference to the result of the comparison by the threshold discriminating section 12a. 
As FIG. 18 illustrates, when the temperature in the apparatus, which is detected by the temperature sensor 20, is not higher than the switching threshold temperature Th1 (=15° C.), the control CPU 12H instructs the enhancing conversion section 14H to select and refer to the OS table memory (ROM) 1a. On this account, the enhancing conversion section 14H performs the enhancing conversion of input image data, with reference to the OS parameter LEVEL 1 stored in the OS table memory (ROM) 1a. 
When the temperature in the apparatus is higher than the switching threshold temperature Th1 (=15° C.) but not higher than the switching threshold temperature Th2 (=25° C.), the control CPU 12H instructs the enhancing conversion section 14H to select and refer to the OS table memory (ROM) 1b. On this account, the enhancing conversion section 14H performs the enhancing conversion of input image data, with reference to the OS parameter LEVEL 2 stored in the OS table memory (ROM) 1b. 
When the temperature in the apparatus is higher than the switching threshold temperature Th2 (=25° C.) but not higher than the switching threshold temperature Th3 (=35° C.), the control CPU 12H instructs the enhancing conversion section 14H to select and refer to the OS table memory (ROM) 1c. On this account, the enhancing conversion section 14H performs the enhancing conversion of input image data, with reference to the OS parameter LEVEL 3 stored in the OS table memory (ROM) 1c. 
When the temperature in the apparatus is higher than the switching threshold temperature Th3 (=35° C.), the control CPU 12H instructs the enhancing conversion section 14H to select and refer to the OS table memory (ROM) 1d. On this account, the enhancing conversion section 14H performs the enhancing conversion of input image data, with reference to the OS parameter LEVEL 4 stored in the OS table memory (ROM) 1d. 
Liquid crystal display panels typically have the following problem: a time required for transit from one halftone to another halftone is long, and the follow-up to an input signal is significantly bad when the temperature is low, so that the response time is long. For this reason, the halftone cannot be reproduced within one frame period (16.7 msec in a case of 60 Hz progressive scanning), and hence an afterimage (image sticking) appears and the halftone is not properly reproduced. Using the aforesaid overshoot drive circuit, a desired halftone can be reproduced within a short period (one frame period) as shown in FIG. 19, by subjecting the gray level of the input image data to the enhancing conversion in the direction of the gray level transition, in such a manner as to cause the brightness of the liquid crystal display panel to attain the brightness corresponding to a target grayscale defined by the input image data, after a predetermined frame period elapses.